The present invention relates to memory address generators, and more particularly to an address generator for a high speed data averager where a repetitive input signal is sampled in an equivalent time mode.
In the digital oscilloscope arts to digitize high frequency signals an equivalent time mode is used. In this mode a sample clock is skewed from a trigger event for successive iterations of a repetitive signal. A memory control circuit adds an increment to the address for each data sample so that the samples for each iteration of the signal occur in a memory spaced apart by the increment. The clock is skewed so that for the next iteration of the signal the data samples fall into the adjacent memory addresses to the prior data samples. The clock is successively skewed for successive iterations of the signal until all the memory locations are filled to give a complete picture of the signal waveform, which may then be displayed. Such an equivalent time mode system is described in U.S. Pat. No. 4,755,951 issued to Allen L. Hollister on Jul. 5, 1988 entitled "Method and Apparatus for Digitizing a Waveform."
The Hollister patent also describes two different modes for combining equivalent time sampling with hardware averaging, a spin mode for high frequency signals and a comb mode for lower frequency signals. In the spin mode the input waveform is sampled only once per repetitive waveform at the same relative location, divided and added to the contents of the current memory address, the address remaining unchanged until all samples are acquired. Then the current memory address is incremented by one and the next sample is taken from a delayed relative location, and the process of sampling, dividing, summing and storing into the current memory address is repeated. This process occurs for several points along the waveform. In the comb mode the input waveform is sampled many times during each repetitive input waveform and each sample is digitized, divided and summed with the contents of the current address which is incremented by the sampling interval for each successive sample. The address is reset to an initial address at the beginning of each waveform and the process is repeated until the necessary number of waveforms have been sampled at several similar points.
In a time domain reflectometer there are two sets of timing pulses generated, a pulse signal and a sample signal. The pulse signal activates an energy source, such as a laser for an optical time domain reflectometer or an electronic pulse circuit for a metallic time domain reflectometer, that transmits an energy pulse into a transmission line, either optical fiber or metallic. The sample signal is used to sample reflections and backscattered energy received from the transmission line in response to the energy pulse within a predetermined acquisition window. In co-pending U.S. patent application Ser. No. 07/399,663, filed Aug. 28, 1989 by William A. Trent entitled "Digital Time Base With Differential Period Delay," the sample clock is phase locked to a master oscillator and remains unchanged, while the pulse signal is delayed from pulse to pulse.
What is desired is an address generator for a high speed averager that applies equivalent time sampling techniques to a time domain reflectometer where the sample signal is not skewed.